A predictive model of the effect of crystal agglomeration on particle form and size distribution requires the quantification of various process parameters that depend on the microscopic properties of specific crystal faces and their interaction with the solvent. In this article, we discuss the various stages in the agglomeration process, using the results of recent experiments on breaking the agglomerative bond and atomic level simulations on the forces involved in crystal aggregation, to highlight the questions that need to be resolved for agglomeration processes to be understood.
A novel experimental apparatus has been developed which enables the measurement of adhesion forces between two crystals suspended in a supersaturated solution and allowed to agglomerate over a fixed time period. The geometry of the crystal surfaces at the contact points and the dynamic development of the bond are captured on video and characterised using an image analysis technique. The experimental apparatus has been designed to allow control of supersaturation, orientation of crystal faces, distance between crystals, relative movement of crystals and contact time. The experimental results show that the agglomerate bond strength, expressed as the agglomerate adhesion force per unit contact area, increases with increasing supersaturation and is higher for faster growing faces than for slower growing faces. In addition, a qualitative comparison has been made between the measured force and a theoretical estimation of the interaction force between crystal faces, determined through molecular modelling. It is shown that the speed of approach of two opposing crystal faces is a key parameter in the nature of the subsequent bond, as is their atomic structure.
In combustion applications of fluidised bed reactors, the solid particles are subject to heterogeneous gas-solid chemical reactions, abrasive attrition and other thermal and mechanical processes. The resulting changes in the overall solid phase significantly influence reactor performance. This paper illustrates a particle balance model which accommodates particle distributions dependent on both size and density as well as populations consisting of multiple solids. The proposed model is tested using literature data on coal conversion obtained in a pilot scale circulating fluidised bed combustor. Model simulations give a fair representation of experimental results for different coal ranks and in a range of operating conditions, including varying temperature of combustion, excess of oxygen and superficial gas velocity in the bed.
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